Sahih Bukhari Volume 007, Book 063, Hadith Number 261.
Narrated By Said bin Jubair : I said to Ibn 'Umar, "If a man accuses his wife of illegal sexual intercourse (what is the judgment)?" He said, "Allah's Prophet separated the couple of Bani 'Ajlan (when the husband accused his wife for an illegal sexual intercourse). The Prophet said, 'Allah knows that one of you two IS a liar; so will one of you repent?' But they refused. He then again said, 'Allah knows that one of you two is a liar; so will one of you repent?' But they refused, whereupon he separated them by divorce." Aiyub (a sub-narrator) said: 'Amr bin Dinar said to me, "In the narration there is something which I do not see you mentioning, i.e. the husband said, "What about my money (Mahr)?' The Prophet said, "You are not entitled to take back money, for if you told the truth you have already entered upon her (and consummated your marriage with her) and if you are a liar then you are less entitled to take it back.